Github user holdenk commented on a diff in the pull request:

    https://github.com/apache/spark/pull/8564#discussion_r39898676
  
    --- Diff: 
mllib/src/main/scala/org/apache/spark/ml/regression/LinearRegression.scala ---
    @@ -298,11 +298,7 @@ class LinearRegressionModel private[ml] (
        */
       // TODO: decide on a good name before exposing to public API
       private[regression] def evaluate(dataset: DataFrame): 
LinearRegressionSummary = {
    -    val t = udf { features: Vector => predict(features) }
    -    val predictionAndObservations = dataset
    -      .select(col($(labelCol)), 
t(col($(featuresCol))).as($(predictionCol)))
    -
    -    new LinearRegressionSummary(predictionAndObservations, 
$(predictionCol), $(labelCol))
    +    new LinearRegressionSummary(transform(dataset), $(predictionCol), 
$(labelCol))
    --- End diff --
    
    I'm not really sure handling linear regression without a prediction column 
makes a lot of sense, or even explicitly checking the predictionCol here. The 
previous version assumed it was set, as do many of the other transformations 
(it also has a default value of "prediction"). I think if the user 
intentionally clears the prediction column or sets it to an invalid column it 
would be better to stick with the existing behaviour and messaging for 
consistency across the methods. That being said I will of course defer to your 
judgement.


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